There are many drugs which are unstable against light and therefore such drugs are put in practical use by shielding light or by a special pharmaceutical design to prevent decomposition during use.
An aqueous preparation of a drug substance is more easily decomposed by light than a solid form, and the decomposition may cause coloring of the aqueous preparation, etc.
In the case of an aqueous preparation such as an ophthalmic solution, it has so far been difficult to solve the problem of decomposition by light by the pharmaceutical design. Therefore, decomposition has been prevented by shielding light.
However, a complete block of light is practically difficult, and thus we have to revert to the improvement of the formulation to stabilize the solution itself.
Ingredients which can be formulated in ophthalmic solutions are restricted because ophthalmic solutions are administered to the eyes, which are highly sensitive organs. Such restriction causes one of the severe obstacles to design photostable ophthalmic solutions.